xref: /openssl/crypto/bn/bn_exp.c (revision af0025fc)
1 /*
2  * Copyright 1995-2023 The OpenSSL Project Authors. All Rights Reserved.
3  *
4  * Licensed under the Apache License 2.0 (the "License").  You may not use
5  * this file except in compliance with the License.  You can obtain a copy
6  * in the file LICENSE in the source distribution or at
7  * https://www.openssl.org/source/license.html
8  */
9 
10 #include "internal/cryptlib.h"
11 #include "internal/constant_time.h"
12 #include "bn_local.h"
13 
14 #include <stdlib.h>
15 #ifdef _WIN32
16 # include <malloc.h>
17 # ifndef alloca
18 #  define alloca _alloca
19 # endif
20 #elif defined(__GNUC__)
21 # ifndef alloca
22 #  define alloca(s) __builtin_alloca((s))
23 # endif
24 #elif defined(__sun)
25 # include <alloca.h>
26 #endif
27 
28 #include "rsaz_exp.h"
29 
30 #undef SPARC_T4_MONT
31 #if defined(OPENSSL_BN_ASM_MONT) && (defined(__sparc__) || defined(__sparc))
32 # include "crypto/sparc_arch.h"
33 # define SPARC_T4_MONT
34 #endif
35 
36 /* maximum precomputation table size for *variable* sliding windows */
37 #define TABLE_SIZE      32
38 
39 /*
40  * Beyond this limit the constant time code is disabled due to
41  * the possible overflow in the computation of powerbufLen in
42  * BN_mod_exp_mont_consttime.
43  * When this limit is exceeded, the computation will be done using
44  * non-constant time code, but it will take very long.
45  */
46 #define BN_CONSTTIME_SIZE_LIMIT (INT_MAX / BN_BYTES / 256)
47 
48 /* this one works - simple but works */
BN_exp(BIGNUM * r,const BIGNUM * a,const BIGNUM * p,BN_CTX * ctx)49 int BN_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
50 {
51     int i, bits, ret = 0;
52     BIGNUM *v, *rr;
53 
54     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
55             || BN_get_flags(a, BN_FLG_CONSTTIME) != 0) {
56         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
57         ERR_raise(ERR_LIB_BN, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
58         return 0;
59     }
60 
61     BN_CTX_start(ctx);
62     rr = ((r == a) || (r == p)) ? BN_CTX_get(ctx) : r;
63     v = BN_CTX_get(ctx);
64     if (rr == NULL || v == NULL)
65         goto err;
66 
67     if (BN_copy(v, a) == NULL)
68         goto err;
69     bits = BN_num_bits(p);
70 
71     if (BN_is_odd(p)) {
72         if (BN_copy(rr, a) == NULL)
73             goto err;
74     } else {
75         if (!BN_one(rr))
76             goto err;
77     }
78 
79     for (i = 1; i < bits; i++) {
80         if (!BN_sqr(v, v, ctx))
81             goto err;
82         if (BN_is_bit_set(p, i)) {
83             if (!BN_mul(rr, rr, v, ctx))
84                 goto err;
85         }
86     }
87     if (r != rr && BN_copy(r, rr) == NULL)
88         goto err;
89 
90     ret = 1;
91  err:
92     BN_CTX_end(ctx);
93     bn_check_top(r);
94     return ret;
95 }
96 
BN_mod_exp(BIGNUM * r,const BIGNUM * a,const BIGNUM * p,const BIGNUM * m,BN_CTX * ctx)97 int BN_mod_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, const BIGNUM *m,
98                BN_CTX *ctx)
99 {
100     int ret;
101 
102     bn_check_top(a);
103     bn_check_top(p);
104     bn_check_top(m);
105 
106     /*-
107      * For even modulus  m = 2^k*m_odd, it might make sense to compute
108      * a^p mod m_odd  and  a^p mod 2^k  separately (with Montgomery
109      * exponentiation for the odd part), using appropriate exponent
110      * reductions, and combine the results using the CRT.
111      *
112      * For now, we use Montgomery only if the modulus is odd; otherwise,
113      * exponentiation using the reciprocal-based quick remaindering
114      * algorithm is used.
115      *
116      * (Timing obtained with expspeed.c [computations  a^p mod m
117      * where  a, p, m  are of the same length: 256, 512, 1024, 2048,
118      * 4096, 8192 bits], compared to the running time of the
119      * standard algorithm:
120      *
121      *   BN_mod_exp_mont   33 .. 40 %  [AMD K6-2, Linux, debug configuration]
122      *                     55 .. 77 %  [UltraSparc processor, but
123      *                                  debug-solaris-sparcv8-gcc conf.]
124      *
125      *   BN_mod_exp_recp   50 .. 70 %  [AMD K6-2, Linux, debug configuration]
126      *                     62 .. 118 % [UltraSparc, debug-solaris-sparcv8-gcc]
127      *
128      * On the Sparc, BN_mod_exp_recp was faster than BN_mod_exp_mont
129      * at 2048 and more bits, but at 512 and 1024 bits, it was
130      * slower even than the standard algorithm!
131      *
132      * "Real" timings [linux-elf, solaris-sparcv9-gcc configurations]
133      * should be obtained when the new Montgomery reduction code
134      * has been integrated into OpenSSL.)
135      */
136 
137 #define MONT_MUL_MOD
138 #define MONT_EXP_WORD
139 #define RECP_MUL_MOD
140 
141 #ifdef MONT_MUL_MOD
142     if (BN_is_odd(m)) {
143 # ifdef MONT_EXP_WORD
144         if (a->top == 1 && !a->neg
145             && (BN_get_flags(p, BN_FLG_CONSTTIME) == 0)
146             && (BN_get_flags(a, BN_FLG_CONSTTIME) == 0)
147             && (BN_get_flags(m, BN_FLG_CONSTTIME) == 0)) {
148             BN_ULONG A = a->d[0];
149             ret = BN_mod_exp_mont_word(r, A, p, m, ctx, NULL);
150         } else
151 # endif
152             ret = BN_mod_exp_mont(r, a, p, m, ctx, NULL);
153     } else
154 #endif
155 #ifdef RECP_MUL_MOD
156     {
157         ret = BN_mod_exp_recp(r, a, p, m, ctx);
158     }
159 #else
160     {
161         ret = BN_mod_exp_simple(r, a, p, m, ctx);
162     }
163 #endif
164 
165     bn_check_top(r);
166     return ret;
167 }
168 
BN_mod_exp_recp(BIGNUM * r,const BIGNUM * a,const BIGNUM * p,const BIGNUM * m,BN_CTX * ctx)169 int BN_mod_exp_recp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
170                     const BIGNUM *m, BN_CTX *ctx)
171 {
172     int i, j, bits, ret = 0, wstart, wend, window;
173     int start = 1;
174     BIGNUM *aa;
175     /* Table of variables obtained from 'ctx' */
176     BIGNUM *val[TABLE_SIZE];
177     BN_RECP_CTX recp;
178 
179     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
180             || BN_get_flags(a, BN_FLG_CONSTTIME) != 0
181             || BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
182         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
183         ERR_raise(ERR_LIB_BN, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
184         return 0;
185     }
186 
187     bits = BN_num_bits(p);
188     if (bits == 0) {
189         /* x**0 mod 1, or x**0 mod -1 is still zero. */
190         if (BN_abs_is_word(m, 1)) {
191             ret = 1;
192             BN_zero(r);
193         } else {
194             ret = BN_one(r);
195         }
196         return ret;
197     }
198 
199     BN_RECP_CTX_init(&recp);
200 
201     BN_CTX_start(ctx);
202     aa = BN_CTX_get(ctx);
203     val[0] = BN_CTX_get(ctx);
204     if (val[0] == NULL)
205         goto err;
206 
207     if (m->neg) {
208         /* ignore sign of 'm' */
209         if (!BN_copy(aa, m))
210             goto err;
211         aa->neg = 0;
212         if (BN_RECP_CTX_set(&recp, aa, ctx) <= 0)
213             goto err;
214     } else {
215         if (BN_RECP_CTX_set(&recp, m, ctx) <= 0)
216             goto err;
217     }
218 
219     if (!BN_nnmod(val[0], a, m, ctx))
220         goto err;               /* 1 */
221     if (BN_is_zero(val[0])) {
222         BN_zero(r);
223         ret = 1;
224         goto err;
225     }
226 
227     window = BN_window_bits_for_exponent_size(bits);
228     if (window > 1) {
229         if (!BN_mod_mul_reciprocal(aa, val[0], val[0], &recp, ctx))
230             goto err;           /* 2 */
231         j = 1 << (window - 1);
232         for (i = 1; i < j; i++) {
233             if (((val[i] = BN_CTX_get(ctx)) == NULL) ||
234                 !BN_mod_mul_reciprocal(val[i], val[i - 1], aa, &recp, ctx))
235                 goto err;
236         }
237     }
238 
239     start = 1;                  /* This is used to avoid multiplication etc
240                                  * when there is only the value '1' in the
241                                  * buffer. */
242     wstart = bits - 1;          /* The top bit of the window */
243     wend = 0;                   /* The bottom bit of the window */
244 
245     if (r == p) {
246         BIGNUM *p_dup = BN_CTX_get(ctx);
247 
248         if (p_dup == NULL || BN_copy(p_dup, p) == NULL)
249             goto err;
250         p = p_dup;
251     }
252 
253     if (!BN_one(r))
254         goto err;
255 
256     for (;;) {
257         int wvalue;             /* The 'value' of the window */
258 
259         if (BN_is_bit_set(p, wstart) == 0) {
260             if (!start)
261                 if (!BN_mod_mul_reciprocal(r, r, r, &recp, ctx))
262                     goto err;
263             if (wstart == 0)
264                 break;
265             wstart--;
266             continue;
267         }
268         /*
269          * We now have wstart on a 'set' bit, we now need to work out how bit
270          * a window to do.  To do this we need to scan forward until the last
271          * set bit before the end of the window
272          */
273         wvalue = 1;
274         wend = 0;
275         for (i = 1; i < window; i++) {
276             if (wstart - i < 0)
277                 break;
278             if (BN_is_bit_set(p, wstart - i)) {
279                 wvalue <<= (i - wend);
280                 wvalue |= 1;
281                 wend = i;
282             }
283         }
284 
285         /* wend is the size of the current window */
286         j = wend + 1;
287         /* add the 'bytes above' */
288         if (!start)
289             for (i = 0; i < j; i++) {
290                 if (!BN_mod_mul_reciprocal(r, r, r, &recp, ctx))
291                     goto err;
292             }
293 
294         /* wvalue will be an odd number < 2^window */
295         if (!BN_mod_mul_reciprocal(r, r, val[wvalue >> 1], &recp, ctx))
296             goto err;
297 
298         /* move the 'window' down further */
299         wstart -= wend + 1;
300         start = 0;
301         if (wstart < 0)
302             break;
303     }
304     ret = 1;
305  err:
306     BN_CTX_end(ctx);
307     BN_RECP_CTX_free(&recp);
308     bn_check_top(r);
309     return ret;
310 }
311 
BN_mod_exp_mont(BIGNUM * rr,const BIGNUM * a,const BIGNUM * p,const BIGNUM * m,BN_CTX * ctx,BN_MONT_CTX * in_mont)312 int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
313                     const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont)
314 {
315     int i, j, bits, ret = 0, wstart, wend, window;
316     int start = 1;
317     BIGNUM *d, *r;
318     const BIGNUM *aa;
319     /* Table of variables obtained from 'ctx' */
320     BIGNUM *val[TABLE_SIZE];
321     BN_MONT_CTX *mont = NULL;
322 
323     bn_check_top(a);
324     bn_check_top(p);
325     bn_check_top(m);
326 
327     if (!BN_is_odd(m)) {
328         ERR_raise(ERR_LIB_BN, BN_R_CALLED_WITH_EVEN_MODULUS);
329         return 0;
330     }
331 
332     if (m->top <= BN_CONSTTIME_SIZE_LIMIT
333         && (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
334             || BN_get_flags(a, BN_FLG_CONSTTIME) != 0
335             || BN_get_flags(m, BN_FLG_CONSTTIME) != 0)) {
336         return BN_mod_exp_mont_consttime(rr, a, p, m, ctx, in_mont);
337     }
338 
339     bits = BN_num_bits(p);
340     if (bits == 0) {
341         /* x**0 mod 1, or x**0 mod -1 is still zero. */
342         if (BN_abs_is_word(m, 1)) {
343             ret = 1;
344             BN_zero(rr);
345         } else {
346             ret = BN_one(rr);
347         }
348         return ret;
349     }
350 
351     BN_CTX_start(ctx);
352     d = BN_CTX_get(ctx);
353     r = BN_CTX_get(ctx);
354     val[0] = BN_CTX_get(ctx);
355     if (val[0] == NULL)
356         goto err;
357 
358     /*
359      * If this is not done, things will break in the montgomery part
360      */
361 
362     if (in_mont != NULL)
363         mont = in_mont;
364     else {
365         if ((mont = BN_MONT_CTX_new()) == NULL)
366             goto err;
367         if (!BN_MONT_CTX_set(mont, m, ctx))
368             goto err;
369     }
370 
371     if (a->neg || BN_ucmp(a, m) >= 0) {
372         if (!BN_nnmod(val[0], a, m, ctx))
373             goto err;
374         aa = val[0];
375     } else
376         aa = a;
377     if (!bn_to_mont_fixed_top(val[0], aa, mont, ctx))
378         goto err;               /* 1 */
379 
380     window = BN_window_bits_for_exponent_size(bits);
381     if (window > 1) {
382         if (!bn_mul_mont_fixed_top(d, val[0], val[0], mont, ctx))
383             goto err;           /* 2 */
384         j = 1 << (window - 1);
385         for (i = 1; i < j; i++) {
386             if (((val[i] = BN_CTX_get(ctx)) == NULL) ||
387                 !bn_mul_mont_fixed_top(val[i], val[i - 1], d, mont, ctx))
388                 goto err;
389         }
390     }
391 
392     start = 1;                  /* This is used to avoid multiplication etc
393                                  * when there is only the value '1' in the
394                                  * buffer. */
395     wstart = bits - 1;          /* The top bit of the window */
396     wend = 0;                   /* The bottom bit of the window */
397 
398 #if 1                           /* by Shay Gueron's suggestion */
399     j = m->top;                 /* borrow j */
400     if (m->d[j - 1] & (((BN_ULONG)1) << (BN_BITS2 - 1))) {
401         if (bn_wexpand(r, j) == NULL)
402             goto err;
403         /* 2^(top*BN_BITS2) - m */
404         r->d[0] = (0 - m->d[0]) & BN_MASK2;
405         for (i = 1; i < j; i++)
406             r->d[i] = (~m->d[i]) & BN_MASK2;
407         r->top = j;
408         r->flags |= BN_FLG_FIXED_TOP;
409     } else
410 #endif
411     if (!bn_to_mont_fixed_top(r, BN_value_one(), mont, ctx))
412         goto err;
413     for (;;) {
414         int wvalue;             /* The 'value' of the window */
415 
416         if (BN_is_bit_set(p, wstart) == 0) {
417             if (!start) {
418                 if (!bn_mul_mont_fixed_top(r, r, r, mont, ctx))
419                     goto err;
420             }
421             if (wstart == 0)
422                 break;
423             wstart--;
424             continue;
425         }
426         /*
427          * We now have wstart on a 'set' bit, we now need to work out how bit
428          * a window to do.  To do this we need to scan forward until the last
429          * set bit before the end of the window
430          */
431         wvalue = 1;
432         wend = 0;
433         for (i = 1; i < window; i++) {
434             if (wstart - i < 0)
435                 break;
436             if (BN_is_bit_set(p, wstart - i)) {
437                 wvalue <<= (i - wend);
438                 wvalue |= 1;
439                 wend = i;
440             }
441         }
442 
443         /* wend is the size of the current window */
444         j = wend + 1;
445         /* add the 'bytes above' */
446         if (!start)
447             for (i = 0; i < j; i++) {
448                 if (!bn_mul_mont_fixed_top(r, r, r, mont, ctx))
449                     goto err;
450             }
451 
452         /* wvalue will be an odd number < 2^window */
453         if (!bn_mul_mont_fixed_top(r, r, val[wvalue >> 1], mont, ctx))
454             goto err;
455 
456         /* move the 'window' down further */
457         wstart -= wend + 1;
458         start = 0;
459         if (wstart < 0)
460             break;
461     }
462     /*
463      * Done with zero-padded intermediate BIGNUMs. Final BN_from_montgomery
464      * removes padding [if any] and makes return value suitable for public
465      * API consumer.
466      */
467 #if defined(SPARC_T4_MONT)
468     if (OPENSSL_sparcv9cap_P[0] & (SPARCV9_VIS3 | SPARCV9_PREFER_FPU)) {
469         j = mont->N.top;        /* borrow j */
470         val[0]->d[0] = 1;       /* borrow val[0] */
471         for (i = 1; i < j; i++)
472             val[0]->d[i] = 0;
473         val[0]->top = j;
474         if (!BN_mod_mul_montgomery(rr, r, val[0], mont, ctx))
475             goto err;
476     } else
477 #endif
478     if (!BN_from_montgomery(rr, r, mont, ctx))
479         goto err;
480     ret = 1;
481  err:
482     if (in_mont == NULL)
483         BN_MONT_CTX_free(mont);
484     BN_CTX_end(ctx);
485     bn_check_top(rr);
486     return ret;
487 }
488 
bn_get_bits(const BIGNUM * a,int bitpos)489 static BN_ULONG bn_get_bits(const BIGNUM *a, int bitpos)
490 {
491     BN_ULONG ret = 0;
492     int wordpos;
493 
494     wordpos = bitpos / BN_BITS2;
495     bitpos %= BN_BITS2;
496     if (wordpos >= 0 && wordpos < a->top) {
497         ret = a->d[wordpos] & BN_MASK2;
498         if (bitpos) {
499             ret >>= bitpos;
500             if (++wordpos < a->top)
501                 ret |= a->d[wordpos] << (BN_BITS2 - bitpos);
502         }
503     }
504 
505     return ret & BN_MASK2;
506 }
507 
508 /*
509  * BN_mod_exp_mont_consttime() stores the precomputed powers in a specific
510  * layout so that accessing any of these table values shows the same access
511  * pattern as far as cache lines are concerned.  The following functions are
512  * used to transfer a BIGNUM from/to that table.
513  */
514 
MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM * b,int top,unsigned char * buf,int idx,int window)515 static int MOD_EXP_CTIME_COPY_TO_PREBUF(const BIGNUM *b, int top,
516                                         unsigned char *buf, int idx,
517                                         int window)
518 {
519     int i, j;
520     int width = 1 << window;
521     BN_ULONG *table = (BN_ULONG *)buf;
522 
523     if (top > b->top)
524         top = b->top;           /* this works because 'buf' is explicitly
525                                  * zeroed */
526     for (i = 0, j = idx; i < top; i++, j += width) {
527         table[j] = b->d[i];
528     }
529 
530     return 1;
531 }
532 
MOD_EXP_CTIME_COPY_FROM_PREBUF(BIGNUM * b,int top,unsigned char * buf,int idx,int window)533 static int MOD_EXP_CTIME_COPY_FROM_PREBUF(BIGNUM *b, int top,
534                                           unsigned char *buf, int idx,
535                                           int window)
536 {
537     int i, j;
538     int width = 1 << window;
539     /*
540      * We declare table 'volatile' in order to discourage compiler
541      * from reordering loads from the table. Concern is that if
542      * reordered in specific manner loads might give away the
543      * information we are trying to conceal. Some would argue that
544      * compiler can reorder them anyway, but it can as well be
545      * argued that doing so would be violation of standard...
546      */
547     volatile BN_ULONG *table = (volatile BN_ULONG *)buf;
548 
549     if (bn_wexpand(b, top) == NULL)
550         return 0;
551 
552     if (window <= 3) {
553         for (i = 0; i < top; i++, table += width) {
554             BN_ULONG acc = 0;
555 
556             for (j = 0; j < width; j++) {
557                 acc |= table[j] &
558                        ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));
559             }
560 
561             b->d[i] = acc;
562         }
563     } else {
564         int xstride = 1 << (window - 2);
565         BN_ULONG y0, y1, y2, y3;
566 
567         i = idx >> (window - 2);        /* equivalent of idx / xstride */
568         idx &= xstride - 1;             /* equivalent of idx % xstride */
569 
570         y0 = (BN_ULONG)0 - (constant_time_eq_int(i,0)&1);
571         y1 = (BN_ULONG)0 - (constant_time_eq_int(i,1)&1);
572         y2 = (BN_ULONG)0 - (constant_time_eq_int(i,2)&1);
573         y3 = (BN_ULONG)0 - (constant_time_eq_int(i,3)&1);
574 
575         for (i = 0; i < top; i++, table += width) {
576             BN_ULONG acc = 0;
577 
578             for (j = 0; j < xstride; j++) {
579                 acc |= ( (table[j + 0 * xstride] & y0) |
580                          (table[j + 1 * xstride] & y1) |
581                          (table[j + 2 * xstride] & y2) |
582                          (table[j + 3 * xstride] & y3) )
583                        & ((BN_ULONG)0 - (constant_time_eq_int(j,idx)&1));
584             }
585 
586             b->d[i] = acc;
587         }
588     }
589 
590     b->top = top;
591     b->flags |= BN_FLG_FIXED_TOP;
592     return 1;
593 }
594 
595 /*
596  * Given a pointer value, compute the next address that is a cache line
597  * multiple.
598  */
599 #define MOD_EXP_CTIME_ALIGN(x_) \
600         ((unsigned char*)(x_) + (MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH - (((size_t)(x_)) & (MOD_EXP_CTIME_MIN_CACHE_LINE_MASK))))
601 
602 /*
603  * This variant of BN_mod_exp_mont() uses fixed windows and the special
604  * precomputation memory layout to limit data-dependency to a minimum to
605  * protect secret exponents (cf. the hyper-threading timing attacks pointed
606  * out by Colin Percival,
607  * http://www.daemonology.net/hyperthreading-considered-harmful/)
608  */
BN_mod_exp_mont_consttime(BIGNUM * rr,const BIGNUM * a,const BIGNUM * p,const BIGNUM * m,BN_CTX * ctx,BN_MONT_CTX * in_mont)609 int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
610                               const BIGNUM *m, BN_CTX *ctx,
611                               BN_MONT_CTX *in_mont)
612 {
613     int i, bits, ret = 0, window, wvalue, wmask, window0;
614     int top;
615     BN_MONT_CTX *mont = NULL;
616 
617     int numPowers;
618     unsigned char *powerbufFree = NULL;
619     int powerbufLen = 0;
620     unsigned char *powerbuf = NULL;
621     BIGNUM tmp, am;
622 #if defined(SPARC_T4_MONT)
623     unsigned int t4 = 0;
624 #endif
625 
626     bn_check_top(a);
627     bn_check_top(p);
628     bn_check_top(m);
629 
630     if (!BN_is_odd(m)) {
631         ERR_raise(ERR_LIB_BN, BN_R_CALLED_WITH_EVEN_MODULUS);
632         return 0;
633     }
634 
635     top = m->top;
636 
637     if (top > BN_CONSTTIME_SIZE_LIMIT) {
638         /* Prevent overflowing the powerbufLen computation below */
639         return BN_mod_exp_mont(rr, a, p, m, ctx, in_mont);
640     }
641 
642     /*
643      * Use all bits stored in |p|, rather than |BN_num_bits|, so we do not leak
644      * whether the top bits are zero.
645      */
646     bits = p->top * BN_BITS2;
647     if (bits == 0) {
648         /* x**0 mod 1, or x**0 mod -1 is still zero. */
649         if (BN_abs_is_word(m, 1)) {
650             ret = 1;
651             BN_zero(rr);
652         } else {
653             ret = BN_one(rr);
654         }
655         return ret;
656     }
657 
658     BN_CTX_start(ctx);
659 
660     /*
661      * Allocate a montgomery context if it was not supplied by the caller. If
662      * this is not done, things will break in the montgomery part.
663      */
664     if (in_mont != NULL)
665         mont = in_mont;
666     else {
667         if ((mont = BN_MONT_CTX_new()) == NULL)
668             goto err;
669         if (!BN_MONT_CTX_set(mont, m, ctx))
670             goto err;
671     }
672 
673     if (a->neg || BN_ucmp(a, m) >= 0) {
674         BIGNUM *reduced = BN_CTX_get(ctx);
675         if (reduced == NULL
676             || !BN_nnmod(reduced, a, m, ctx)) {
677             goto err;
678         }
679         a = reduced;
680     }
681 
682 #ifdef RSAZ_ENABLED
683     /*
684      * If the size of the operands allow it, perform the optimized
685      * RSAZ exponentiation. For further information see
686      * crypto/bn/rsaz_exp.c and accompanying assembly modules.
687      */
688     if ((16 == a->top) && (16 == p->top) && (BN_num_bits(m) == 1024)
689         && rsaz_avx2_eligible()) {
690         if (NULL == bn_wexpand(rr, 16))
691             goto err;
692         RSAZ_1024_mod_exp_avx2(rr->d, a->d, p->d, m->d, mont->RR.d,
693                                mont->n0[0]);
694         rr->top = 16;
695         rr->neg = 0;
696         bn_correct_top(rr);
697         ret = 1;
698         goto err;
699     } else if ((8 == a->top) && (8 == p->top) && (BN_num_bits(m) == 512)) {
700         if (NULL == bn_wexpand(rr, 8))
701             goto err;
702         RSAZ_512_mod_exp(rr->d, a->d, p->d, m->d, mont->n0[0], mont->RR.d);
703         rr->top = 8;
704         rr->neg = 0;
705         bn_correct_top(rr);
706         ret = 1;
707         goto err;
708     }
709 #endif
710 
711     /* Get the window size to use with size of p. */
712     window = BN_window_bits_for_ctime_exponent_size(bits);
713 #if defined(SPARC_T4_MONT)
714     if (window >= 5 && (top & 15) == 0 && top <= 64 &&
715         (OPENSSL_sparcv9cap_P[1] & (CFR_MONTMUL | CFR_MONTSQR)) ==
716         (CFR_MONTMUL | CFR_MONTSQR) && (t4 = OPENSSL_sparcv9cap_P[0]))
717         window = 5;
718     else
719 #endif
720 #if defined(OPENSSL_BN_ASM_MONT5)
721     if (window >= 5 && top <= BN_SOFT_LIMIT) {
722         window = 5;             /* ~5% improvement for RSA2048 sign, and even
723                                  * for RSA4096 */
724         /* reserve space for mont->N.d[] copy */
725         powerbufLen += top * sizeof(mont->N.d[0]);
726     }
727 #endif
728     (void)0;
729 
730     /*
731      * Allocate a buffer large enough to hold all of the pre-computed powers
732      * of am, am itself and tmp.
733      */
734     numPowers = 1 << window;
735     powerbufLen += sizeof(m->d[0]) * (top * numPowers +
736                                       ((2 * top) >
737                                        numPowers ? (2 * top) : numPowers));
738 #ifdef alloca
739     if (powerbufLen < 3072)
740         powerbufFree =
741             alloca(powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH);
742     else
743 #endif
744         if ((powerbufFree =
745              OPENSSL_malloc(powerbufLen + MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH))
746             == NULL)
747         goto err;
748 
749     powerbuf = MOD_EXP_CTIME_ALIGN(powerbufFree);
750     memset(powerbuf, 0, powerbufLen);
751 
752 #ifdef alloca
753     if (powerbufLen < 3072)
754         powerbufFree = NULL;
755 #endif
756 
757     /* lay down tmp and am right after powers table */
758     tmp.d = (BN_ULONG *)(powerbuf + sizeof(m->d[0]) * top * numPowers);
759     am.d = tmp.d + top;
760     tmp.top = am.top = 0;
761     tmp.dmax = am.dmax = top;
762     tmp.neg = am.neg = 0;
763     tmp.flags = am.flags = BN_FLG_STATIC_DATA;
764 
765     /* prepare a^0 in Montgomery domain */
766 #if 1                           /* by Shay Gueron's suggestion */
767     if (m->d[top - 1] & (((BN_ULONG)1) << (BN_BITS2 - 1))) {
768         /* 2^(top*BN_BITS2) - m */
769         tmp.d[0] = (0 - m->d[0]) & BN_MASK2;
770         for (i = 1; i < top; i++)
771             tmp.d[i] = (~m->d[i]) & BN_MASK2;
772         tmp.top = top;
773     } else
774 #endif
775     if (!bn_to_mont_fixed_top(&tmp, BN_value_one(), mont, ctx))
776         goto err;
777 
778     /* prepare a^1 in Montgomery domain */
779     if (!bn_to_mont_fixed_top(&am, a, mont, ctx))
780         goto err;
781 
782     if (top > BN_SOFT_LIMIT)
783         goto fallback;
784 
785 #if defined(SPARC_T4_MONT)
786     if (t4) {
787         typedef int (*bn_pwr5_mont_f) (BN_ULONG *tp, const BN_ULONG *np,
788                                        const BN_ULONG *n0, const void *table,
789                                        int power, int bits);
790         int bn_pwr5_mont_t4_8(BN_ULONG *tp, const BN_ULONG *np,
791                               const BN_ULONG *n0, const void *table,
792                               int power, int bits);
793         int bn_pwr5_mont_t4_16(BN_ULONG *tp, const BN_ULONG *np,
794                                const BN_ULONG *n0, const void *table,
795                                int power, int bits);
796         int bn_pwr5_mont_t4_24(BN_ULONG *tp, const BN_ULONG *np,
797                                const BN_ULONG *n0, const void *table,
798                                int power, int bits);
799         int bn_pwr5_mont_t4_32(BN_ULONG *tp, const BN_ULONG *np,
800                                const BN_ULONG *n0, const void *table,
801                                int power, int bits);
802         static const bn_pwr5_mont_f pwr5_funcs[4] = {
803             bn_pwr5_mont_t4_8, bn_pwr5_mont_t4_16,
804             bn_pwr5_mont_t4_24, bn_pwr5_mont_t4_32
805         };
806         bn_pwr5_mont_f pwr5_worker = pwr5_funcs[top / 16 - 1];
807 
808         typedef int (*bn_mul_mont_f) (BN_ULONG *rp, const BN_ULONG *ap,
809                                       const void *bp, const BN_ULONG *np,
810                                       const BN_ULONG *n0);
811         int bn_mul_mont_t4_8(BN_ULONG *rp, const BN_ULONG *ap, const void *bp,
812                              const BN_ULONG *np, const BN_ULONG *n0);
813         int bn_mul_mont_t4_16(BN_ULONG *rp, const BN_ULONG *ap,
814                               const void *bp, const BN_ULONG *np,
815                               const BN_ULONG *n0);
816         int bn_mul_mont_t4_24(BN_ULONG *rp, const BN_ULONG *ap,
817                               const void *bp, const BN_ULONG *np,
818                               const BN_ULONG *n0);
819         int bn_mul_mont_t4_32(BN_ULONG *rp, const BN_ULONG *ap,
820                               const void *bp, const BN_ULONG *np,
821                               const BN_ULONG *n0);
822         static const bn_mul_mont_f mul_funcs[4] = {
823             bn_mul_mont_t4_8, bn_mul_mont_t4_16,
824             bn_mul_mont_t4_24, bn_mul_mont_t4_32
825         };
826         bn_mul_mont_f mul_worker = mul_funcs[top / 16 - 1];
827 
828         void bn_mul_mont_vis3(BN_ULONG *rp, const BN_ULONG *ap,
829                               const void *bp, const BN_ULONG *np,
830                               const BN_ULONG *n0, int num);
831         void bn_mul_mont_t4(BN_ULONG *rp, const BN_ULONG *ap,
832                             const void *bp, const BN_ULONG *np,
833                             const BN_ULONG *n0, int num);
834         void bn_mul_mont_gather5_t4(BN_ULONG *rp, const BN_ULONG *ap,
835                                     const void *table, const BN_ULONG *np,
836                                     const BN_ULONG *n0, int num, int power);
837         void bn_flip_n_scatter5_t4(const BN_ULONG *inp, size_t num,
838                                    void *table, size_t power);
839         void bn_gather5_t4(BN_ULONG *out, size_t num,
840                            void *table, size_t power);
841         void bn_flip_t4(BN_ULONG *dst, BN_ULONG *src, size_t num);
842 
843         BN_ULONG *np = mont->N.d, *n0 = mont->n0;
844         int stride = 5 * (6 - (top / 16 - 1)); /* multiple of 5, but less
845                                                 * than 32 */
846 
847         /*
848          * BN_to_montgomery can contaminate words above .top [in
849          * BN_DEBUG build...
850          */
851         for (i = am.top; i < top; i++)
852             am.d[i] = 0;
853         for (i = tmp.top; i < top; i++)
854             tmp.d[i] = 0;
855 
856         bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, 0);
857         bn_flip_n_scatter5_t4(am.d, top, powerbuf, 1);
858         if (!(*mul_worker) (tmp.d, am.d, am.d, np, n0) &&
859             !(*mul_worker) (tmp.d, am.d, am.d, np, n0))
860             bn_mul_mont_vis3(tmp.d, am.d, am.d, np, n0, top);
861         bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, 2);
862 
863         for (i = 3; i < 32; i++) {
864             /* Calculate a^i = a^(i-1) * a */
865             if (!(*mul_worker) (tmp.d, tmp.d, am.d, np, n0) &&
866                 !(*mul_worker) (tmp.d, tmp.d, am.d, np, n0))
867                 bn_mul_mont_vis3(tmp.d, tmp.d, am.d, np, n0, top);
868             bn_flip_n_scatter5_t4(tmp.d, top, powerbuf, i);
869         }
870 
871         /* switch to 64-bit domain */
872         np = alloca(top * sizeof(BN_ULONG));
873         top /= 2;
874         bn_flip_t4(np, mont->N.d, top);
875 
876         /*
877          * The exponent may not have a whole number of fixed-size windows.
878          * To simplify the main loop, the initial window has between 1 and
879          * full-window-size bits such that what remains is always a whole
880          * number of windows
881          */
882         window0 = (bits - 1) % 5 + 1;
883         wmask = (1 << window0) - 1;
884         bits -= window0;
885         wvalue = bn_get_bits(p, bits) & wmask;
886         bn_gather5_t4(tmp.d, top, powerbuf, wvalue);
887 
888         /*
889          * Scan the exponent one window at a time starting from the most
890          * significant bits.
891          */
892         while (bits > 0) {
893             if (bits < stride)
894                 stride = bits;
895             bits -= stride;
896             wvalue = bn_get_bits(p, bits);
897 
898             if ((*pwr5_worker) (tmp.d, np, n0, powerbuf, wvalue, stride))
899                 continue;
900             /* retry once and fall back */
901             if ((*pwr5_worker) (tmp.d, np, n0, powerbuf, wvalue, stride))
902                 continue;
903 
904             bits += stride - 5;
905             wvalue >>= stride - 5;
906             wvalue &= 31;
907             bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
908             bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
909             bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
910             bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
911             bn_mul_mont_t4(tmp.d, tmp.d, tmp.d, np, n0, top);
912             bn_mul_mont_gather5_t4(tmp.d, tmp.d, powerbuf, np, n0, top,
913                                    wvalue);
914         }
915 
916         bn_flip_t4(tmp.d, tmp.d, top);
917         top *= 2;
918         /* back to 32-bit domain */
919         tmp.top = top;
920         bn_correct_top(&tmp);
921         OPENSSL_cleanse(np, top * sizeof(BN_ULONG));
922     } else
923 #endif
924 #if defined(OPENSSL_BN_ASM_MONT5)
925     if (window == 5 && top > 1) {
926         /*
927          * This optimization uses ideas from https://eprint.iacr.org/2011/239,
928          * specifically optimization of cache-timing attack countermeasures,
929          * pre-computation optimization, and Almost Montgomery Multiplication.
930          *
931          * The paper discusses a 4-bit window to optimize 512-bit modular
932          * exponentiation, used in RSA-1024 with CRT, but RSA-1024 is no longer
933          * important.
934          *
935          * |bn_mul_mont_gather5| and |bn_power5| implement the "almost"
936          * reduction variant, so the values here may not be fully reduced.
937          * They are bounded by R (i.e. they fit in |top| words), not |m|.
938          * Additionally, we pass these "almost" reduced inputs into
939          * |bn_mul_mont|, which implements the normal reduction variant.
940          * Given those inputs, |bn_mul_mont| may not give reduced
941          * output, but it will still produce "almost" reduced output.
942          */
943         void bn_mul_mont_gather5(BN_ULONG *rp, const BN_ULONG *ap,
944                                  const void *table, const BN_ULONG *np,
945                                  const BN_ULONG *n0, int num, int power);
946         void bn_scatter5(const BN_ULONG *inp, size_t num,
947                          void *table, size_t power);
948         void bn_gather5(BN_ULONG *out, size_t num, void *table, size_t power);
949         void bn_power5(BN_ULONG *rp, const BN_ULONG *ap,
950                        const void *table, const BN_ULONG *np,
951                        const BN_ULONG *n0, int num, int power);
952         int bn_get_bits5(const BN_ULONG *ap, int off);
953 
954         BN_ULONG *n0 = mont->n0, *np;
955 
956         /*
957          * BN_to_montgomery can contaminate words above .top [in
958          * BN_DEBUG build...
959          */
960         for (i = am.top; i < top; i++)
961             am.d[i] = 0;
962         for (i = tmp.top; i < top; i++)
963             tmp.d[i] = 0;
964 
965         /*
966          * copy mont->N.d[] to improve cache locality
967          */
968         for (np = am.d + top, i = 0; i < top; i++)
969             np[i] = mont->N.d[i];
970 
971         bn_scatter5(tmp.d, top, powerbuf, 0);
972         bn_scatter5(am.d, am.top, powerbuf, 1);
973         bn_mul_mont(tmp.d, am.d, am.d, np, n0, top);
974         bn_scatter5(tmp.d, top, powerbuf, 2);
975 
976 # if 0
977         for (i = 3; i < 32; i++) {
978             /* Calculate a^i = a^(i-1) * a */
979             bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
980             bn_scatter5(tmp.d, top, powerbuf, i);
981         }
982 # else
983         /* same as above, but uses squaring for 1/2 of operations */
984         for (i = 4; i < 32; i *= 2) {
985             bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
986             bn_scatter5(tmp.d, top, powerbuf, i);
987         }
988         for (i = 3; i < 8; i += 2) {
989             int j;
990             bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
991             bn_scatter5(tmp.d, top, powerbuf, i);
992             for (j = 2 * i; j < 32; j *= 2) {
993                 bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
994                 bn_scatter5(tmp.d, top, powerbuf, j);
995             }
996         }
997         for (; i < 16; i += 2) {
998             bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
999             bn_scatter5(tmp.d, top, powerbuf, i);
1000             bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
1001             bn_scatter5(tmp.d, top, powerbuf, 2 * i);
1002         }
1003         for (; i < 32; i += 2) {
1004             bn_mul_mont_gather5(tmp.d, am.d, powerbuf, np, n0, top, i - 1);
1005             bn_scatter5(tmp.d, top, powerbuf, i);
1006         }
1007 # endif
1008         /*
1009          * The exponent may not have a whole number of fixed-size windows.
1010          * To simplify the main loop, the initial window has between 1 and
1011          * full-window-size bits such that what remains is always a whole
1012          * number of windows
1013          */
1014         window0 = (bits - 1) % 5 + 1;
1015         wmask = (1 << window0) - 1;
1016         bits -= window0;
1017         wvalue = bn_get_bits(p, bits) & wmask;
1018         bn_gather5(tmp.d, top, powerbuf, wvalue);
1019 
1020         /*
1021          * Scan the exponent one window at a time starting from the most
1022          * significant bits.
1023          */
1024         if (top & 7) {
1025             while (bits > 0) {
1026                 bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
1027                 bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
1028                 bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
1029                 bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
1030                 bn_mul_mont(tmp.d, tmp.d, tmp.d, np, n0, top);
1031                 bn_mul_mont_gather5(tmp.d, tmp.d, powerbuf, np, n0, top,
1032                                     bn_get_bits5(p->d, bits -= 5));
1033             }
1034         } else {
1035             while (bits > 0) {
1036                 bn_power5(tmp.d, tmp.d, powerbuf, np, n0, top,
1037                           bn_get_bits5(p->d, bits -= 5));
1038             }
1039         }
1040 
1041         tmp.top = top;
1042         /*
1043          * The result is now in |tmp| in Montgomery form, but it may not be
1044          * fully reduced. This is within bounds for |BN_from_montgomery|
1045          * (tmp < R <= m*R) so it will, when converting from Montgomery form,
1046          * produce a fully reduced result.
1047          *
1048          * This differs from Figure 2 of the paper, which uses AMM(h, 1) to
1049          * convert from Montgomery form with unreduced output, followed by an
1050          * extra reduction step. In the paper's terminology, we replace
1051          * steps 9 and 10 with MM(h, 1).
1052          */
1053     } else
1054 #endif
1055     {
1056  fallback:
1057         if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, window))
1058             goto err;
1059         if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, window))
1060             goto err;
1061 
1062         /*
1063          * If the window size is greater than 1, then calculate
1064          * val[i=2..2^winsize-1]. Powers are computed as a*a^(i-1) (even
1065          * powers could instead be computed as (a^(i/2))^2 to use the slight
1066          * performance advantage of sqr over mul).
1067          */
1068         if (window > 1) {
1069             if (!bn_mul_mont_fixed_top(&tmp, &am, &am, mont, ctx))
1070                 goto err;
1071             if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 2,
1072                                               window))
1073                 goto err;
1074             for (i = 3; i < numPowers; i++) {
1075                 /* Calculate a^i = a^(i-1) * a */
1076                 if (!bn_mul_mont_fixed_top(&tmp, &am, &tmp, mont, ctx))
1077                     goto err;
1078                 if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, i,
1079                                                   window))
1080                     goto err;
1081             }
1082         }
1083 
1084         /*
1085          * The exponent may not have a whole number of fixed-size windows.
1086          * To simplify the main loop, the initial window has between 1 and
1087          * full-window-size bits such that what remains is always a whole
1088          * number of windows
1089          */
1090         window0 = (bits - 1) % window + 1;
1091         wmask = (1 << window0) - 1;
1092         bits -= window0;
1093         wvalue = bn_get_bits(p, bits) & wmask;
1094         if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&tmp, top, powerbuf, wvalue,
1095                                             window))
1096             goto err;
1097 
1098         wmask = (1 << window) - 1;
1099         /*
1100          * Scan the exponent one window at a time starting from the most
1101          * significant bits.
1102          */
1103         while (bits > 0) {
1104 
1105             /* Square the result window-size times */
1106             for (i = 0; i < window; i++)
1107                 if (!bn_mul_mont_fixed_top(&tmp, &tmp, &tmp, mont, ctx))
1108                     goto err;
1109 
1110             /*
1111              * Get a window's worth of bits from the exponent
1112              * This avoids calling BN_is_bit_set for each bit, which
1113              * is not only slower but also makes each bit vulnerable to
1114              * EM (and likely other) side-channel attacks like One&Done
1115              * (for details see "One&Done: A Single-Decryption EM-Based
1116              *  Attack on OpenSSL's Constant-Time Blinded RSA" by M. Alam,
1117              *  H. Khan, M. Dey, N. Sinha, R. Callan, A. Zajic, and
1118              *  M. Prvulovic, in USENIX Security'18)
1119              */
1120             bits -= window;
1121             wvalue = bn_get_bits(p, bits) & wmask;
1122             /*
1123              * Fetch the appropriate pre-computed value from the pre-buf
1124              */
1125             if (!MOD_EXP_CTIME_COPY_FROM_PREBUF(&am, top, powerbuf, wvalue,
1126                                                 window))
1127                 goto err;
1128 
1129             /* Multiply the result into the intermediate result */
1130             if (!bn_mul_mont_fixed_top(&tmp, &tmp, &am, mont, ctx))
1131                 goto err;
1132         }
1133     }
1134 
1135     /*
1136      * Done with zero-padded intermediate BIGNUMs. Final BN_from_montgomery
1137      * removes padding [if any] and makes return value suitable for public
1138      * API consumer.
1139      */
1140 #if defined(SPARC_T4_MONT)
1141     if (OPENSSL_sparcv9cap_P[0] & (SPARCV9_VIS3 | SPARCV9_PREFER_FPU)) {
1142         am.d[0] = 1;            /* borrow am */
1143         for (i = 1; i < top; i++)
1144             am.d[i] = 0;
1145         if (!BN_mod_mul_montgomery(rr, &tmp, &am, mont, ctx))
1146             goto err;
1147     } else
1148 #endif
1149     if (!BN_from_montgomery(rr, &tmp, mont, ctx))
1150         goto err;
1151     ret = 1;
1152  err:
1153     if (in_mont == NULL)
1154         BN_MONT_CTX_free(mont);
1155     if (powerbuf != NULL) {
1156         OPENSSL_cleanse(powerbuf, powerbufLen);
1157         OPENSSL_free(powerbufFree);
1158     }
1159     BN_CTX_end(ctx);
1160     return ret;
1161 }
1162 
BN_mod_exp_mont_word(BIGNUM * rr,BN_ULONG a,const BIGNUM * p,const BIGNUM * m,BN_CTX * ctx,BN_MONT_CTX * in_mont)1163 int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p,
1164                          const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *in_mont)
1165 {
1166     BN_MONT_CTX *mont = NULL;
1167     int b, bits, ret = 0;
1168     int r_is_one;
1169     BN_ULONG w, next_w;
1170     BIGNUM *r, *t;
1171     BIGNUM *swap_tmp;
1172 #define BN_MOD_MUL_WORD(r, w, m) \
1173                 (BN_mul_word(r, (w)) && \
1174                 (/* BN_ucmp(r, (m)) < 0 ? 1 :*/  \
1175                         (BN_mod(t, r, m, ctx) && (swap_tmp = r, r = t, t = swap_tmp, 1))))
1176     /*
1177      * BN_MOD_MUL_WORD is only used with 'w' large, so the BN_ucmp test is
1178      * probably more overhead than always using BN_mod (which uses BN_copy if
1179      * a similar test returns true).
1180      */
1181     /*
1182      * We can use BN_mod and do not need BN_nnmod because our accumulator is
1183      * never negative (the result of BN_mod does not depend on the sign of
1184      * the modulus).
1185      */
1186 #define BN_TO_MONTGOMERY_WORD(r, w, mont) \
1187                 (BN_set_word(r, (w)) && BN_to_montgomery(r, r, (mont), ctx))
1188 
1189     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
1190             || BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
1191         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
1192         ERR_raise(ERR_LIB_BN, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
1193         return 0;
1194     }
1195 
1196     bn_check_top(p);
1197     bn_check_top(m);
1198 
1199     if (!BN_is_odd(m)) {
1200         ERR_raise(ERR_LIB_BN, BN_R_CALLED_WITH_EVEN_MODULUS);
1201         return 0;
1202     }
1203     if (m->top == 1)
1204         a %= m->d[0];           /* make sure that 'a' is reduced */
1205 
1206     bits = BN_num_bits(p);
1207     if (bits == 0) {
1208         /* x**0 mod 1, or x**0 mod -1 is still zero. */
1209         if (BN_abs_is_word(m, 1)) {
1210             ret = 1;
1211             BN_zero(rr);
1212         } else {
1213             ret = BN_one(rr);
1214         }
1215         return ret;
1216     }
1217     if (a == 0) {
1218         BN_zero(rr);
1219         ret = 1;
1220         return ret;
1221     }
1222 
1223     BN_CTX_start(ctx);
1224     r = BN_CTX_get(ctx);
1225     t = BN_CTX_get(ctx);
1226     if (t == NULL)
1227         goto err;
1228 
1229     if (in_mont != NULL)
1230         mont = in_mont;
1231     else {
1232         if ((mont = BN_MONT_CTX_new()) == NULL)
1233             goto err;
1234         if (!BN_MONT_CTX_set(mont, m, ctx))
1235             goto err;
1236     }
1237 
1238     r_is_one = 1;               /* except for Montgomery factor */
1239 
1240     /* bits-1 >= 0 */
1241 
1242     /* The result is accumulated in the product r*w. */
1243     w = a;                      /* bit 'bits-1' of 'p' is always set */
1244     for (b = bits - 2; b >= 0; b--) {
1245         /* First, square r*w. */
1246         next_w = w * w;
1247         if ((next_w / w) != w) { /* overflow */
1248             if (r_is_one) {
1249                 if (!BN_TO_MONTGOMERY_WORD(r, w, mont))
1250                     goto err;
1251                 r_is_one = 0;
1252             } else {
1253                 if (!BN_MOD_MUL_WORD(r, w, m))
1254                     goto err;
1255             }
1256             next_w = 1;
1257         }
1258         w = next_w;
1259         if (!r_is_one) {
1260             if (!BN_mod_mul_montgomery(r, r, r, mont, ctx))
1261                 goto err;
1262         }
1263 
1264         /* Second, multiply r*w by 'a' if exponent bit is set. */
1265         if (BN_is_bit_set(p, b)) {
1266             next_w = w * a;
1267             if ((next_w / a) != w) { /* overflow */
1268                 if (r_is_one) {
1269                     if (!BN_TO_MONTGOMERY_WORD(r, w, mont))
1270                         goto err;
1271                     r_is_one = 0;
1272                 } else {
1273                     if (!BN_MOD_MUL_WORD(r, w, m))
1274                         goto err;
1275                 }
1276                 next_w = a;
1277             }
1278             w = next_w;
1279         }
1280     }
1281 
1282     /* Finally, set r:=r*w. */
1283     if (w != 1) {
1284         if (r_is_one) {
1285             if (!BN_TO_MONTGOMERY_WORD(r, w, mont))
1286                 goto err;
1287             r_is_one = 0;
1288         } else {
1289             if (!BN_MOD_MUL_WORD(r, w, m))
1290                 goto err;
1291         }
1292     }
1293 
1294     if (r_is_one) {             /* can happen only if a == 1 */
1295         if (!BN_one(rr))
1296             goto err;
1297     } else {
1298         if (!BN_from_montgomery(rr, r, mont, ctx))
1299             goto err;
1300     }
1301     ret = 1;
1302  err:
1303     if (in_mont == NULL)
1304         BN_MONT_CTX_free(mont);
1305     BN_CTX_end(ctx);
1306     bn_check_top(rr);
1307     return ret;
1308 }
1309 
1310 /* The old fallback, simple version :-) */
BN_mod_exp_simple(BIGNUM * r,const BIGNUM * a,const BIGNUM * p,const BIGNUM * m,BN_CTX * ctx)1311 int BN_mod_exp_simple(BIGNUM *r, const BIGNUM *a, const BIGNUM *p,
1312                       const BIGNUM *m, BN_CTX *ctx)
1313 {
1314     int i, j, bits, ret = 0, wstart, wend, window;
1315     int start = 1;
1316     BIGNUM *d;
1317     /* Table of variables obtained from 'ctx' */
1318     BIGNUM *val[TABLE_SIZE];
1319 
1320     if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
1321             || BN_get_flags(a, BN_FLG_CONSTTIME) != 0
1322             || BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
1323         /* BN_FLG_CONSTTIME only supported by BN_mod_exp_mont() */
1324         ERR_raise(ERR_LIB_BN, ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED);
1325         return 0;
1326     }
1327 
1328     if (r == m) {
1329         ERR_raise(ERR_LIB_BN, ERR_R_PASSED_INVALID_ARGUMENT);
1330         return 0;
1331     }
1332 
1333     bits = BN_num_bits(p);
1334     if (bits == 0) {
1335         /* x**0 mod 1, or x**0 mod -1 is still zero. */
1336         if (BN_abs_is_word(m, 1)) {
1337             ret = 1;
1338             BN_zero(r);
1339         } else {
1340             ret = BN_one(r);
1341         }
1342         return ret;
1343     }
1344 
1345     BN_CTX_start(ctx);
1346     d = BN_CTX_get(ctx);
1347     val[0] = BN_CTX_get(ctx);
1348     if (val[0] == NULL)
1349         goto err;
1350 
1351     if (!BN_nnmod(val[0], a, m, ctx))
1352         goto err;               /* 1 */
1353     if (BN_is_zero(val[0])) {
1354         BN_zero(r);
1355         ret = 1;
1356         goto err;
1357     }
1358 
1359     window = BN_window_bits_for_exponent_size(bits);
1360     if (window > 1) {
1361         if (!BN_mod_mul(d, val[0], val[0], m, ctx))
1362             goto err;           /* 2 */
1363         j = 1 << (window - 1);
1364         for (i = 1; i < j; i++) {
1365             if (((val[i] = BN_CTX_get(ctx)) == NULL) ||
1366                 !BN_mod_mul(val[i], val[i - 1], d, m, ctx))
1367                 goto err;
1368         }
1369     }
1370 
1371     start = 1;                  /* This is used to avoid multiplication etc
1372                                  * when there is only the value '1' in the
1373                                  * buffer. */
1374     wstart = bits - 1;          /* The top bit of the window */
1375     wend = 0;                   /* The bottom bit of the window */
1376 
1377     if (r == p) {
1378         BIGNUM *p_dup = BN_CTX_get(ctx);
1379 
1380         if (p_dup == NULL || BN_copy(p_dup, p) == NULL)
1381             goto err;
1382         p = p_dup;
1383     }
1384 
1385     if (!BN_one(r))
1386         goto err;
1387 
1388     for (;;) {
1389         int wvalue;             /* The 'value' of the window */
1390 
1391         if (BN_is_bit_set(p, wstart) == 0) {
1392             if (!start)
1393                 if (!BN_mod_mul(r, r, r, m, ctx))
1394                     goto err;
1395             if (wstart == 0)
1396                 break;
1397             wstart--;
1398             continue;
1399         }
1400         /*
1401          * We now have wstart on a 'set' bit, we now need to work out how bit
1402          * a window to do.  To do this we need to scan forward until the last
1403          * set bit before the end of the window
1404          */
1405         wvalue = 1;
1406         wend = 0;
1407         for (i = 1; i < window; i++) {
1408             if (wstart - i < 0)
1409                 break;
1410             if (BN_is_bit_set(p, wstart - i)) {
1411                 wvalue <<= (i - wend);
1412                 wvalue |= 1;
1413                 wend = i;
1414             }
1415         }
1416 
1417         /* wend is the size of the current window */
1418         j = wend + 1;
1419         /* add the 'bytes above' */
1420         if (!start)
1421             for (i = 0; i < j; i++) {
1422                 if (!BN_mod_mul(r, r, r, m, ctx))
1423                     goto err;
1424             }
1425 
1426         /* wvalue will be an odd number < 2^window */
1427         if (!BN_mod_mul(r, r, val[wvalue >> 1], m, ctx))
1428             goto err;
1429 
1430         /* move the 'window' down further */
1431         wstart -= wend + 1;
1432         start = 0;
1433         if (wstart < 0)
1434             break;
1435     }
1436     ret = 1;
1437  err:
1438     BN_CTX_end(ctx);
1439     bn_check_top(r);
1440     return ret;
1441 }
1442 
1443 /*
1444  * This is a variant of modular exponentiation optimization that does
1445  * parallel 2-primes exponentiation using 256-bit (AVX512VL) AVX512_IFMA ISA
1446  * in 52-bit binary redundant representation.
1447  * If such instructions are not available, or input data size is not supported,
1448  * it falls back to two BN_mod_exp_mont_consttime() calls.
1449  */
BN_mod_exp_mont_consttime_x2(BIGNUM * rr1,const BIGNUM * a1,const BIGNUM * p1,const BIGNUM * m1,BN_MONT_CTX * in_mont1,BIGNUM * rr2,const BIGNUM * a2,const BIGNUM * p2,const BIGNUM * m2,BN_MONT_CTX * in_mont2,BN_CTX * ctx)1450 int BN_mod_exp_mont_consttime_x2(BIGNUM *rr1, const BIGNUM *a1, const BIGNUM *p1,
1451                                  const BIGNUM *m1, BN_MONT_CTX *in_mont1,
1452                                  BIGNUM *rr2, const BIGNUM *a2, const BIGNUM *p2,
1453                                  const BIGNUM *m2, BN_MONT_CTX *in_mont2,
1454                                  BN_CTX *ctx)
1455 {
1456     int ret = 0;
1457 
1458 #ifdef RSAZ_ENABLED
1459     BN_MONT_CTX *mont1 = NULL;
1460     BN_MONT_CTX *mont2 = NULL;
1461 
1462     if (ossl_rsaz_avx512ifma_eligible() &&
1463         (((a1->top == 16) && (p1->top == 16) && (BN_num_bits(m1) == 1024) &&
1464           (a2->top == 16) && (p2->top == 16) && (BN_num_bits(m2) == 1024)) ||
1465          ((a1->top == 24) && (p1->top == 24) && (BN_num_bits(m1) == 1536) &&
1466           (a2->top == 24) && (p2->top == 24) && (BN_num_bits(m2) == 1536)) ||
1467          ((a1->top == 32) && (p1->top == 32) && (BN_num_bits(m1) == 2048) &&
1468           (a2->top == 32) && (p2->top == 32) && (BN_num_bits(m2) == 2048)))) {
1469 
1470         int topn = a1->top;
1471         /* Modulus bits of |m1| and |m2| are equal */
1472         int mod_bits = BN_num_bits(m1);
1473 
1474         if (bn_wexpand(rr1, topn) == NULL)
1475             goto err;
1476         if (bn_wexpand(rr2, topn) == NULL)
1477             goto err;
1478 
1479         /*  Ensure that montgomery contexts are initialized */
1480         if (in_mont1 != NULL) {
1481             mont1 = in_mont1;
1482         } else {
1483             if ((mont1 = BN_MONT_CTX_new()) == NULL)
1484                 goto err;
1485             if (!BN_MONT_CTX_set(mont1, m1, ctx))
1486                 goto err;
1487         }
1488         if (in_mont2 != NULL) {
1489             mont2 = in_mont2;
1490         } else {
1491             if ((mont2 = BN_MONT_CTX_new()) == NULL)
1492                 goto err;
1493             if (!BN_MONT_CTX_set(mont2, m2, ctx))
1494                 goto err;
1495         }
1496 
1497         ret = ossl_rsaz_mod_exp_avx512_x2(rr1->d, a1->d, p1->d, m1->d,
1498                                           mont1->RR.d, mont1->n0[0],
1499                                           rr2->d, a2->d, p2->d, m2->d,
1500                                           mont2->RR.d, mont2->n0[0],
1501                                           mod_bits);
1502 
1503         rr1->top = topn;
1504         rr1->neg = 0;
1505         bn_correct_top(rr1);
1506         bn_check_top(rr1);
1507 
1508         rr2->top = topn;
1509         rr2->neg = 0;
1510         bn_correct_top(rr2);
1511         bn_check_top(rr2);
1512 
1513         goto err;
1514     }
1515 #endif
1516 
1517     /* rr1 = a1^p1 mod m1 */
1518     ret = BN_mod_exp_mont_consttime(rr1, a1, p1, m1, ctx, in_mont1);
1519     /* rr2 = a2^p2 mod m2 */
1520     ret &= BN_mod_exp_mont_consttime(rr2, a2, p2, m2, ctx, in_mont2);
1521 
1522 #ifdef RSAZ_ENABLED
1523 err:
1524     if (in_mont2 == NULL)
1525         BN_MONT_CTX_free(mont2);
1526     if (in_mont1 == NULL)
1527         BN_MONT_CTX_free(mont1);
1528 #endif
1529 
1530     return ret;
1531 }
1532